This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9921344.9 filed in Great Britian on Sep. 9, 1999; the entire content of which is hereby incorporated by reference.
This invention relates to an amplifer circuit, and in particular to an amplifier circuit for use in a radio transmitter, more specifically in a TDMA system.
In many radio communications systems, it is necessary to be able to control accurately the power of an output signal which is to be transmitted.
Conventionally, this can be achieved by applying feedback control to the final power amplifier stage in the transmitter. For example, the current flowing in the power amplifier can be taken as a measure of the output power. This current can be detected by a current sensing amplifier, and the result compared with an input reference signal. Any error can then be supplied to a control input of the power amplifier to adjust the output thereof.
However, this conventional technique can only provide accurate control of the output power if the current sensing amplifier is accurate. If there is any offset in the current sensing amplifier, the output signal will not be controlled accurately.
U.S. Pat. No. 5,304,947 discloses a transmitter circuit for a TDMA system. A logic circuit is provided to determine a point at which the transmitter is between transmission time slots. At that point, application of a control signal provides that an offset cancellation value is updated, and applied during the subsequent transmission time slot. However, this requires an additional input to the transmitter circuit.
According to the present invention, any offset in a current sensing amplifier can be cancelled. In the case of a TDMA transmitter, an input signal will be zero at certain times, for example during time slots when the device is not transmitting. The circuit of the present invention detects time periods during which the input signal falls below a threshold level, and adjusts the current sensing amplifier to cancel any offset during such time periods.
This allows accurate measurement of currents in the power amplifier, and hence accurate control of transmitter powers.
Compared with an alternative in which an externally applied control signal is used to determine when offset cancellation should be applied, the present invention requires fewer input control signals, and hence fewer input pins for the circuit.